Conventionally, there has been known an air flow rate measuring device that measures a flow rate of an intake air taking advantage of heat transfer with air while detecting a plurality of physical quantities of the intake air. In an engine system, advanced engine control has been required in order to achieve low fuel consumption. Thus, high accurate measuring of physical quantities of the intake air, such as an absolute humidity, a temperature, a pressure, or the like, is required in addition to measuring of a flow rate of the intake air. These physical quantities are important information involving combustion process of the engine system and are used to calculate, e.g., a fuel injection time.
For example, in a flow rate measuring device disclosed in Patent Literature 1 (JP 2011-075357 A) or Patent Literature 2 (JP 2015-004556 A), a humidity sensor is provided to calculate an absolute humidity of an intake air. The humidity sensor includes a relative humidity detector that detects a relative humidity and a temperature detector that calculates the absolute humidity based on the relative humidity. Furthermore, in addition to the temperature detector (in other words, a first temperature detector), a second temperature detector, which is dedicated to detect a temperature of the intake air, is disposed at a position where the intake air is directly brought into contact with the second temperature detector.
In the configuration described in Patent Literature 1 and Patent Literature 2, the humidity sensor includes the relative humidity detector and the first temperature detector to calculate an absolute humidity of the intake air. The first temperature detector of the humidity sensor takes in an intake air through a sub passage branching inside the air flow rate measuring device, and the first temperature detector is arranged at a position where the intake air is not directly brought into contact with the first temperature detector. Therefore, the first temperature detector is not able to detect a real temperature of the intake air, and thus the second temperature detector is additionally arranged at a position where the intake air is directly brought into contact with the second temperature detector. As a result, two temperature signals are output from the first and second temperature detectors. Hence, the second temperature detector is additionally necessary as an intake air temperature measuring device that dedicatedly a temperature of an intake air. As a result, the number of components such as terminals and harnesses is increased, which would lead to complexity to the device.
Furthermore, the relative humidity detector of the humidity sensor is arranged at a position where the intake air is not brought into contact with the relative humidity detector. Therefore, a relative humidity of the intake air cannot be detected accurately, and as a result, accuracy of calculation of the absolute humidity of the intake air would be deteriorated due to a measuring error of the relative humidity.
In view of the above, it is an objective of the present disclosure to provide an air flow rate measuring device that accurately performs calculation of an absolute humidity of an intake air and measuring of a temperature of the intake air without an additional temperature sensor dedicated to detect a temperature of the intake air.